Roles of MIR-155 in microglia and neurogenesis in response to toll-like receptor mediated inflammation.

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https://hdl.handle.net/2144/10996

Abstract

Adult neurogenesis is a highly plastic process-exhibiting enhancement by
rewarding experiences and suppression by stressful experiences. Animal models have
demonstrated that pro-inflammatory challenges suppress neurogenesis. A growing body
of evidence suggests a role for microRNAs (miRNA) in the regulation of the acute
inflammatory response in peripheral macrophages. However, little is known about the
role miRNAs play in microglia, the CNS resident immune cell , or their impact on
neurogenesis. To elucidate the role that miRNAs have on influencing the acute
inflammatory response, primary murine microglia were stimulated with
lipopolysaccharide (LPS), an M1-classically activating stimulus, and profiled for gene
and miRNA expression. miR-155 and Interleukin-6 (IL-6) were the most up-regulated
miRNA and mRNA respectively, identified under M1-classical activation when
compared to resting microglia. IL-6, the founding member of the neuropoietic family of
cytokines, is a known negative regulator of neurogenesis. The ability of IL-6 to regulate
neurogenesis was tested with the application of an IL-6 neutralizing antibody in a
transwell co-culture system with neural stem cells (NSC) and primary microglia. During
a pro-inflammatory challenge, NSCs altered their differentiation pattern with a 20%
increase in astrocytic and a 50% reduction in neuronal differentiation. These alterations
were greatly reduced with treatment by an IL-6 neutralizing antibody. To test if miR-155
can regulate IL-6levels both miR-15s-1- mice and anti-miR-155 inhibitors were employed
in vitro. Knockout and knockdown of miR-155 demonstrated a 60% and 50% reduction
in IL-6 expression, respectively, when compared to wildtype LPS challenged controls.
To determine if miR-155 has the ability to regulate the observed suppression of
neurogenesis after inflammation, miR-15s-1- mice were subjected to intracerebroventricular injection of LPS. The pro-inflammatory induced suppression of neurogenesis along the Sub Granular Zone (SGZ) was completely ablated in miR-155_1_mice. Specifically, proliferation and neuronal differentiation were both suppressed by ~30% along the SGZ in wildtype animals given a pro-inflammatory challenge. In a similar pro-inflammatory challenge in miR-155_1_ mice, these levels were restored to unchallenged wildtype levels. This work demonstrates that miR-155 has the ability to suppress neurogenesis observed after inflammation, potentially through mediation of neuropoietic cytokine levels e.g. IL-6.